This invention relates to coatings compositions and, more particularly, relates to water-reduced coatings compositions for metal containers.
As is well known, cans which may be made of steel, tin, aluminum and even combinations of these materials have been, and continue to be, used as containers for food and beverages (beer, soda pop, etc.). In order to protect the contents of these containers from contamination by the container metals, it is required that the interiors of such containers be coated with a coating which is essentially inert to each container's contents and which provides an effective barrier between the container and its contents, some of which can be quite acidic.
At present, organic solvent-based epoxy-urea coatings are primarily used as interior can coatings. Use of these coatings requires that the containers first be subjected to a cleaning operation in order to prepare the surfaces of the container for application of the coating. This cleaning step is in addition to the usual cleaning operation employed to remove dirt and oil. The surface preparation cleaning solutions presently employed are principally chromium-based compositions which present a very real water pollution problem when they are disposed of. After the containers have been cleaned as described, an exterior coating is usually applied to beverage containers followed by an inking step which serves to label the containers. These steps are accompanied by separate baking cycles to cure both the base coating and inking materials. Only after all of these steps have been completed is the interior of the container coated.
The presently-employed can coating method which has been described is characterized by several disadvantages, one of which (the chromium cleaning procedure) has already been referred to. A second disadvantage results from the use of the organic solvents which form a volatile component which is evaporated from the film-forming composition during cure of the latter. In order to meet various pollution standards, the organic solvent gases produced during cure must be channeled through an afterburner in order to convert them to less harmful elements. The use of afterburners itself presents an energy conservation problem due to our limited energy resources.
Food containers are usually covered with a paper label rather than the ink label hereinbefore referred to. The paper label is placed over the exterior of the can after the latter has been subjected only to the chromium-cleaning step. However, if the cans are made of steel, they will rust in time. Even if the steel cans are tin clad (for purposes of facilitating drawing the metal to form cans), localized rusting may still occur. Therefore, it would be advantageous to eliminate this rusting problem by use of a coating which is easily and inexpensively applied.